As a method of measuring a gloss of an object surface, a specular glossiness measurement method (JIS-Z8741) has been widely known in which an amount of light reflected is measured in a specular direction with light emitted to the surface to obtain a gloss level. Such a specular glossiness measurement method represents an index indicating only an intensity of a beam reflected in the specular direction, a specular glossiness measured by the specular glossiness measurement method and visual glossiness subjectively evaluated by human are not always conforms with each other.
Thus, another method is proposed which measures light reflected from a plurality of angles to obtain an evaluation value closer to the subjective human evaluation value. For example, there is a method for obtaining an evaluation value about gloss of an object with reference to a gloss index value obtained based on specular light from an object and index values about luminance and saturation based on diffuse reflection light (see Patent Literature 1). Moreover, there is a method for obtaining an evaluation value of glossiness of an object, with reference to a gloss variation evaluation value obtained based on a luminance component of specular light, a gloss reference value obtained based on luminance components of specular light and diffuse reflection light, and a chromaticity variation index value about an amount of variation in color of specular light and diffuse reflection light (see Patent Literature 2).
Usually, a gloss evaluation apparatus for evaluating a gloss of an object is formed to output an identical evaluation value (measurement value) for identical object, for shipment.
However, the gloss evaluation apparatus used for a long time may output different evaluation values for the identical object. This case may be caused by for example temporal change of a light source for emitting light for gloss measurement, or a light receiving element for receiving reflected light. Further, a light source or a light receiving element may have an individual difference in intrinsic characteristic. Still further, a use environment thereof, for example, an ambient temperature may cause variation in evaluation value.